Solid freeform fabrication is a process for manufacturing three-dimensional objects, for example, prototype parts, production parts, models and working tools. Solid freeform fabrication is an additive process in which an object, which is described by electronic data, is automatically built, usually layer-by-layer, from base materials.
There are a variety of different freeform fabrication systems that use different processes to form a desired object. Some systems eject the base material from which the object being formed is built up layer-by-layer. Other systems eject a binder selectively into a powdered base material.
Many solid freeform fabrication systems require the construction of support structures that must be attached below or beside the object being formed. These structures support the object being formed until it is completed and solidified. For example, stereolithography, polymer jetting, jetted wax, and fused deposition systems can all require one or more support structures to be fabricated as part of the build process.
The support structures of the systems mentioned above, however, create several issues. One of the issues associated with the support structures of current systems is the difficulty in removing that support structure from the object being fabricated after the fabrication process is complete. The support structures may be physically connected to the object being formed. Consequently, the process of removing the support structures may damage the object being formed.
Another issue with current systems is the general requirement for support structures to be relatively dense. Support structures range from polymer jetting, where every voxel of the object is supported, to stereolithography, where a matrix of relatively dense support structures is built to support the object in a liquid bath. These dense support structures can be both expensive and time consuming to fabricate.
However, another family of solid freeform fabrication systems is powder-based. Powder-based systems generally create objects by ejecting a binder onto a flat bed of powdered build material. Each powder layer may be dispensed or spread as a dry powder or a slurry within a build area. Wherever a binder is selectively ejected into the powder layer, the powder is bound into a cross section or layer of the object being formed. It is also possible to use focused energy to bind the powder together. Selective laser sintering (SLS) is an example of this approach.
In a powder-based system, unbound powder acts as the primary support for the object being formed. This is advantageous compared to the systems mentioned above, because no explicit support structures are needed. Thus, there are no support structures to remove, no material is wasted to create the support structures, and no machine throughput availability is sacrificed for building support structures. In addition, all of the unbound powder can be removed and reused.
However, one of the issues with powder-based systems is the difficulty in achieving tightly packed powder in the build regions of the fabrication machines. Many factors may cause the powder to shift or settle after layers of powder are spread. Some of the factors that cause powder shifting and/or settling may include mechanical vibration of the system equipment, spreading of subsequent layers, and settling of the powder under the weight of an object being produced in the build area.
When powder shifts and/or settles during the fabrication process, the object being formed will also sink or move. The object may move in any number of directions. This movement causes a loss of dimensional accuracy, as the fabrication systems have no way to measure or compensate for object movement during the fabrication process. If, for example, an object sinks as powder settles, a layer spread after the settling will be thicker than specified. As the fabrication process continues, the overall thickness of the object being formed becomes larger than intended, and the accuracy of the object dimensions is compromised.